The present invention relates to magnesium-based alloys suitable for applications at temperatures as high as 250-300xc2x0 C., which alloys have good mechanical properties, corrosion resistance, and castability.
Magnesium alloys, being the lightest structural metal material, are very attractive in automotive and aerospace industries. New alloys are required that would resist the increasingly onerous operating environment, and that would provide more complex components with increased lifetime and reduced maintenance costs.
An ideal alloy should meet several conditions related to its behavior both during its casting and during its use under continued stress. The good castability includes good flow of melted alloy into thin mold sections, low sticking of the melted alloy to the mold, and resistance to oxidation during the casting process. The alloy should not develop cracks during cooling and solidifying stage of casting. The parts that are cast of the alloy should have high tensile and compressive yield strength, and during their usage they should show a low continued strain under stress at elevated temperatures (creep resistance). The alloy should be further resistant to the corrosion. Some applications, for example use as parts of the gear-box or a crankcase, require that the resistance to corrosion and to mechanical stress be also kept at high temperatures.
The physical and chemical properties of the alloy depend substantially on the presence of other metallic elements, which can form a variety of intermetallic compounds, conferring on the alloy improved mechanical and chemical properties. The selection of elements and their ratio in the alloy is important also from the economic viewpoint, since the cost of the alloy represents a significant part of the total component cost.
Magnesium alloys can conveniently be categorized into two groups, namely Mgxe2x80x94Al based alloys and Mgxe2x80x94Zr based alloys. The best known representative of Mgxe2x80x94Al group is alloy AZ91E which is widely used due to its good castability and good corrosion resistance. However, this alloy has decreased strength and creep resistance above 120xc2x0 C. In addition, the outcropping microporosity followed by lack of pressure tightness is often present in castings, and the mechanical properties of said alloy can vary with section thickness. The mentioned drawbacks, characteristic for Mgxe2x80x94Al alloys, are overcome in Mgxe2x80x94Zr alloys. Zirconium exhibits a potent grain refining effect on magnesium, leading to the greater casting integrity, and improved mechanical properties. Mgxe2x80x94Zr alloys have more consistent properties through thin and thick sections, and are not prone to outcropping through-wall porosity, which prevents lubricant leakage. A variety of alloys, exploiting the unique effect of zirconium, have been developed, some being based on the mixture Mgxe2x80x94Zrxe2x80x94Znxe2x80x94RE (rare earth elements), wherein RE is usually a rare earth mixture with cerium as the major component, others being based on the mixture Mgxe2x80x94Zrxe2x80x94Ndxe2x80x94Ag. Commercial magnesium alloys of the former group, like ZE41 and EZ33, provide moderate strength at ambient temperature with retention of properties up to 150xc2x0 C. Alloys of the latter group, like QE22, can be solution heat-treated and artificially aged to give high strength at temperatures both ambient and higher than 150xc2x0 C. However, both mentioned groups of alloys exhibit poor corrosion resistance due to the presence of 2-5% Zn or 1.5-2.5% Ag. In addition, silver is an expensive element.
Trying to improve existing alloys, yttrium was introduced as a major alloying element. It was found that the presence of yttrium considerably improved the high-temperature properties of the alloys. British patent No. 1,463,609 describes magnesium-based alloy containing 2.5 to 7 wt % yttrium, 1.25 to 3 wt % silver, 0.5 to 3 wt % rare earth metals, 0 to 1 wt % zirconium, 0 to 0.5 wt % zinc, and optionally other components. U.S. Pat. No. 4,194,908 discloses magnesium-based alloys containing 0.1 to 2.5 wt % yttrium, 1.6 to 3.5 wt % silver, 0.1 to 2.3 wt % rare earth metals of which at least 60% is neodymium, and optionally other elements. The patent demonstrates that an improved creep resistance at elevated temperatures could be obtained by the addition of smaller quantities of yttrium to magnesium alloys containing silver and neodymium. When the yttrium content is less than 0.5 wt %, thorium should be present too. However, thorium is radioactive, and its use in magnesium alloys is prohibited. U.S. Pat. No. 3,419,385 discloses magnesium-based alloy which comprises 0.2 to 10 wt % yttrium, 0.5 to 2 wt % silver, 0.1 to 6 wt % zinc, and possibly manganese and zirconium. The alloys of this invention are mostly designated for extrusions. In sand casting, the alloys of this invention are inferior than conventional alloys like QE22. The American patent U.S. Pat. No. 4,116,731 discloses magnesium-based alloys, exhibiting high temperature stability, which are heat treated and aged and which do without silver, said alloys containing 0.8 to 6.0 wt % yttrium, 0.5 to 4 wt % neodymium, 0.1 to 2.2 wt % zinc, 0.31 to 1.1 wt % zirconium, up to 0.05 wt % copper and up to 0.2 wt % manganese, provided that no less than 50% of the total amount of neodymium and yttrium additions enters the solid solution after heat treatment. U.S. Pat. No. 4,401,621 discloses magnesium-based alloys which comprise 1.5 to 10% of yttrium component of which at least 60% is yttrium and the balance are heavy RE metals, 1 to 6 wt % of neodymium component of which at least 60% is neodymium, and possibly other elements, including up to 1% silver. The alloys of said patent exhibit better creep properties than any conventional magnesium alloys including QE22, EZ33, ZE41 and ZC63 alloys, and in addition they have a good corrosion resistance. However, the high content of yttrium makes the alloys too expensive. Moreover, these alloys exhibit worse castability, particularly fluidity, since yttrium increases viscosity of the molten magnesium.
It is therefore an object of this invention to provide magnesium alloys suitable for long-term applications up to 250xc2x0 C. and short-term applications up to 300xc2x0 C. which have good castability.
It is an object of this invention to provide magnesium-based alloys suitable for use sand casting, permanent mold casting, and direct chill casting with subsequent extrusion or/and forging.
It is also an object of this invention to provide alloys, which are well adapted for plastic forming operations such as forging and extrusion.
It is another object of the present invention to provide alloys, which exhibit excellent combination of strength, creep resistance and corrosion resistance.
It is a further object of this invention to provide alloys, which exhibit low corrosion fatigue.
It is still a further object of this invention to provide alloys, which exhibit the aforesaid behavior and properties, and have a relatively low cost, particularly in comparison with commercial magnesium alloys of the types of WE43 or WE54.
Other objects and advantages of the present invention will appear as the description proceeds.
The present invention relates to magnesium-based alloys suitable for applications at temperatures as high as 250-300xc2x0 C. which have good mechanical properties, corrosion resistance, and castability. Said alloys contain at least 92 wt % magnesium, and 2.7 to 3.3 wt % neodymium, 0.0 to 2.6 wt % yttrium, 0.2 to 0.8 wt % zirconium, 0.2 to 0.8 wt % zinc, 0.03 to 0.25 wt % calcium, and 0.00 to 0.001 wt % beryllium. The contents of iron, nickel, copper, and silicon are not higher than 0.007 wt %, 0.002 wt %, 0.003 wt %, and 0.01 wt % respectively. A preferred ratio between yttrium and neodymium contents is from 0.45 to 0.70, and a preferred zirconium content is calculated according to the following equation:
1.3/(wt % Nd+wt % Y)xe2x89xa6wt % Zrxe2x89xa60.6 wt % 
The alloys of this invention are well adopted for sand casting, permanent mold casting, and direct chill casting with subsequent extrusion or/and forging.
The invention further relates to articles produced by casting and forming magnesium-based alloys having the properties defined hereinbefore.